The drive for acquiring good performance in bandwidth and power efficiency of a modulation system is continuous. As far as digital communications are concerned, digital modulation schemes are needed. The prior art in this field is as follows. Before the 1960's, at which time digital communications were only limited to laboratory testings, radio engineers considered offset QPSK (or OQPSK, Offset Quadrature Phase Shift Keying) as the most important digital modulation scheme. In 1961, minimum shift keying (MSK) modulation was introduced in U.S. Pat. No. 2,977,417 as an alternative method for OQPSK. OQPSK and MSK are also disclosed in IEEE Trans. on Communications, Vol. COM-24, No. 8 pp. 809-820, August 1976, by S. A. Gronemeyer & A. L. McBride. Both MSK and OQPSK modulation are still very widely used in many applications.
Concerning the bandwidth efficiency, neither OQPSK nor MSK can compete with multiple level phase, amplitude or their hybrid modulation schemes, such as quadrature amplitude modulation QAM, and combined amplitude-phase shift keying CAPSK. The multiple level phase, amplitude or their hybrid modulations, however, need a relatively high signal-to-noise ratio (SNR) margin to ensure an acceptable detection efficiency; otherwise they might be subject to a rapid degeneration of performance in a noisy channel. Therefore, they are not power efficient modulation schemes. Furthermore, the multiple level phase, amplitude or their hybrid modulations might not be suitable for certain applications, such as applications for non-linear channels.
On the other hand, PSK-type modulations, such as OQPSK and MSK, have a relatively high power efficiency, or robustness against channel noise; the only weakness in PSK-type modulations is their relatively low bandwidth efficiency.
If we define the PSD (power spectral density) main lobe for an OQPSK signal as the unit-normalized bandwidth unit fT in an unit of Hz/bit/sec, then the PSD main lobe for the MSK signal is 1.5 Hz/bit/sec. The most serious problem in OQPSK and MSK modulations is that their PSD side lobes decrease with the frequency at a rather slow rate, approximately at 1/f.sup.2 and 1/f.sup.4 for OQPSK and MSK, respectively. In most applications, these PSD side lobe reduction rates are not sufficient to limit the intersymbol interference (ISI) level under a certain bandwidth, as reported in ICC 1981, pp. 33.4.1-33.4.5, by T. Le-Ngoc & K. Feher. In this sense, both OQPSK and MSK modulations are not very bandwidth-efficient.
In order to increase bandwidth efficiency, quadrature-overlapped raised cosine (QORC) modulation was introduced in the 1980's, as disclosed in IEEE Trans. on Communications, Vol. COM-29, No. 3 pp. 237-249, March 1981, by M. C. Austin & M. U. Chang. OQRC modulation is different from OQPSK and MSK modulations in that it uses pulse overlapping technique to shape the baseband signal before carrier modulation. Therefore, it can use a wider pulse-shaping waveform in the time domain than the OQPSK and MSK modulations under the same data rate, in order to reduce the width of the signal spectrum in the frequency domain. The PSD side lobes for the QORC signal decrease as fast as 1/f.sup.6, faster than in OQPSK or MSK. This means that, under the same system bandwidth, QORC modulation will introduce a lower ISI level than OQPSK and MSK, or under the same ISI requirement, QORC modulation needs less bandwidth allocation than OQPSK and MSK modulations. This entails an improvement in bandwidth efficiency.
The disadvantages of the prior art are as follows: The power efficiency of the prior PSK-type modulations, such as OQPSK and MSK, is high, but their bandwidth efficiency is low. For the multiple level phase, amplitude or their hybrid modulations, the bandwidth efficiency is high, but the power efficiency is low. Further, when QORC modulation is considered, the bandwidth efficiency is certainly improved, but the envelope of the modulated signal is non-constant, and the signal will suffer from BER degrading (power efficiency loss) when applied to non-linear channels, such as satellite communications or terrestrial microwave relay systems which use TWTA as transmitter amplifiers.